Neat or fiber reinforced epoxy resins cure to form highly crosslinked materials characterized by brittle nature and poor thermal conductivity. As a consequence, said materials are limited in their use in many engineering applications, for example in electrical engineering, automotive engineering, and also in construction industry, where special performances, such as good mechanical, thermal and electrical properties, are required. In order to improve the strength, the damage tolerance and the toughness of the parts made from such materials, many efforts have been made to toughen epoxies. For example, amorphous materials with high-temperature resistance such as functionalized poly(aryl ether sulfone)s (PAES) have been used as excellent alternatives of rubbers to toughen multifunctional epoxy resins for applications such as notably, military and civilian aircraft exterior and interior parts, automotive, rail, wind power generation, pressure vessels, motor cases, oil well lining tubes, power transmission tubes, printed circuit boards, and molding compounds.
For example, EP 0486044 A2 demonstrates damage resistance of resin compositions made with epoxy resins toughened by addition of thermoplastic or reactive aromatic oligomer. The examples are in particular related to epoxy resin formulations made from VICTREX® 5003P polyethersulfone or from an amine-terminated polyethersulfone oligomer.
US 2005/0038170 A1 discloses molding compositions comprising epoxy resins and poly(aryl ether sulfone)s containing side or end groups selected from the group consisting of carboxy and anhydride groups. Said molding compositions have improved transparency.
JP 3068896B describes epoxy resin compositions comprising a polyfunctional epoxy compound, a phenolic novolak resin, an inorganic filler, in particular crystalline silica powder, a cure accelerator, in particular 2-methylimidazole and a polyethersulfone having terminal amine groups or terminal phenol groups.
Functionalized poly(aryl ether sulfone)s, featuring reactive end groups are known to possess a high solubility and having improved interfacial properties in epoxy resins. High molecular weight functionalized PAES have been used in the past as epoxies toughener because of their particular effectiveness. However, the viscosity of high molecular weight functionalized PAES toughened epoxy resin mixture prior to curing is also much higher compared to said lower molecular weight PAES toughened epoxy resin mixture, thereby suffering from some drawbacks such a less effective working and curing characteristics of said epoxy resin mixtures. For example, high resin viscosity makes it more difficult to wet the carbon fiber matrix to make notably pre-impregnated materials (prepregs) which are used to form composite parts.
There is thus a need of new poly(aryl ether sulfone)s toughened epoxy resin compositions which can overcome all these drawbacks, mentioned above, and exhibit all the beneficial performance properties, such as high temperature performance, solvent resistance, dimensional stability, and high compressive strength of the high molecular weight PAES toughened epoxy resins while maintaining acceptable low resin viscosity and good processability (e.g. good curability). Said epoxy resin compositions can then efficiently be added to fiber or fabric reinforcement to provide prepregs which can then be molded and cured using autoclave or out-of-autoclave processes to form composite parts and laminates, such as aerospace composite parts, having high strength, improved damage tolerance and improved toughness, in particular fracture toughness. Said epoxy resin compositions can then suitably be used in wet resin processes such as resin transfer molding (RTM), vacuum assisted RTM (VARTM), vacuum bag curing, press molding, Seeman Composite resin infusion (SCRIMP™), Quickstep™, resin infusion under flexible tooling (RIFT), filament winding, pultrusion, and thermal expansion molding.